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How to get the windows username automatically?

I'm developing an intranet web portal for our company, using Angular 5, Java Spring and REST, JDBC, JSON, MS SQL Server. I need seamless authentication to my website for which I've developed a method using LDAP, which retrieves the required details using the user's usename. Now, my only task is to get the username from the user automatically. Please help me to proceed further. Thank You!!

The current user's username is available through a system property:
String username = System.getProperty("user.name");
That's the user running the Java process.
If you're looking for the user running the browser, then you can't get the OS username using JavaScript.

Tomcat Authentication using LDAP with just username and without password

I have a web application which is hosted on apache web server. And i have external java application which runs on tomcat integrated in this web application.
The apache is kerberized and I get the username of the person accessing it in the intranet. I want to use the same username and pass it on tomcat to check against ldap and authorize the user.
I was looking into JNDI realm which supports LDAP module. But the problem is I do not have access to the user's password. But I'm able to get the groups the user belongs to and based on that I want to authenticate the user.
So my question is will i be able to authenticate a user against Tomcat server via LDAP with just the username and not the password?
PS: Sorry there is no code or configuration that I can post at this point. The only resources i found implemented both username and password. I cannot kerberize the tomcat server because I have other issues in the network.

"I was looking into JNDI realm which supports LDAP module. But the problem is I do not have
access to the user's password. But I'm able to get the groups the user belongs to and based
on that I want to authenticate the user."
Once I was asked to write some code to do the very same thing. I found a way to make it work. However, although it seemed to work well enough with Internet Explorer which would automatically detect the Windows username (because my code was parsing some NTLM information that only Internet Explorer passes in as a header), in Firefox or any other browser the user would be presented with an authentication box and any username they typed in would be accepted! Its very insecure.
(Well, its not really true that only IE passes the header in. Only IE passes it in automatically; but Firefox also passes it in after someone gets the authentication box and types in any name they want. That's the problem.)
What I would suggest is writing a C#.NET service to do the authentication for real on a webserver that is IIS and has Integrated Authentication on. Then, redirect to the C# service when someone hits the Java site and the username session variable is null. Have the C# service save the info in a trusted/secure database including ip address and browser and redirect to the Java app which reads the db to validate IP address and browser and that the record was just created.
Edit: I just noticed you said your Apache is kerberized and already gets the Windows username, which may render the problem above about the unreliability of parsing the NTLM header null and void. In this case, if you can get the AD groups via LDAP you can authenticate the user in your webapp with your own code, but probably not using the official Tomcat authentication scheme. What is described above is just because my Tomcat is not paired with an Apache server but with a IIS server. So basically, its the same thing, except you won't need C#.

How to authenticate with Active Directory using java in XPages

I need to know if a user entered correct login information to Microsoft Active directory
I want the user to enter username and password on an xpage and it should only return if these values was entered correctly or not.
I know I can use ldap requests and send in credentials but have not seen any working solutions for this.
I need to use java as the code should be able to run on non windows servers

There are a number of approaches you can take:
The easiest would be to link the AD to Domino using DirectoryAssistance and let users login using their AD credentials in the first place.
If IIS is running on the AD server you could send an HTTP request using the Apache HTTPClient (and basic authentication) and see what's coming back
The best working Java libraries are listed in the Apache Directory project. There you would do an LDAP query (e.g. the user herself)
Let us know which one works for you

You can use JNDI to access the Active Directory. There are a lot of ressources on the web on how to do this, for example have a look at http://mhimu.wordpress.com/2009/03/18/active-directory-authentication-using-javajndi/
You have tagged the question as XPage, do you want to do the authentication from inside domino? In that case the domino server could do the authentication.

Windows Authentication for Java Based web applications, How to?

I have a couple of Java-based web applications developed. Both the applications have separate Authentication logic based on some ActiveX directory implementation.
Now, I need to change this to Windows authentication so that whenever the user hits the URLs of my web applications, instead of redirecting him to login page I need to check his Windows credentials.
I do not want to store his windows credentials in URL.
Is there any good way to do this ?

Depending on the level of integration you want your web application to have, Spring Security should have you covered in just about all aspects of what you are after.
If redirecting to a login page and authenticating the entered credentials against an Active Directory server via LDAP is acceptable, then the LDAP extension is the way to go.
If you want more of a Single Sign On (SSO) flow and your users are already authenticated against the authoritative Active Directory server in question (eg. they are logged in to the domain), then the Kerberos plugin for Spring Security may be more appealing, since your users will simply have to go to the web application and won't have to go through any other authentication steps. The systems will take care of it behind the scenes.
You can also combine / layer these approaches if you which and try Kerberos-based authentication first and if that falls through, fall back to a login form and LDAP-based authentication.
If you need to go beyond that, Spring Security is flexible enough to allow you to use OpenID or in-app authentication as well if needed.

I'd recommending using Active Directory to expose it's windows authentication layer over LDAP, which can then be hit by something like Spring Security.
This would effectively force anyone using your application to use their windows login.

Securing username/password embedded in Java Desktop App

My Java desktop application includes a component for communicating with a web service.
We therefore need to include the access details for it within the application, but do not want it to be easily accessible in the event that the code is decompiled (we will be obfuscating).
What techniques can we use to secure these details?

Do not bother encrypting the password in your application. Whatever you do, a determined user will be able to decrypt it and get access to it. My recommendation is to have a username and password for every user. The application will ask the user to enter the credentials and store them (using MD5 for example). If you can't modify the web service to authenticate many users, create a proxy service that can do that. The proxy service, deployed on a secure environment, will be allowed to have access to the username and password of the secured service.

I prefer you try Java Properties API.